Ile structure of a cloned Drosophila nuclear protein which can catalyze the decondensation of demembranated Xenopus sperm chromatin (chromatin remodeling protein, CRP 1) was studied in the STEM to compare it to Xenopus nucleoplasmin to which it is quite homologous in amino acid sequence. While most evidence suggests that Xenopus nucleoplasmin is pentameric, mass analysis from the STEM suggests that CRP I is tetrameric (while Xenopus nucleoplasmin is pentameric), in agreement with chemical crosslinking experiments. However, negatively stained CRP I and Xenopus nucleoplasmin were morphologically indistinguishable in the STEM. (Crevel et al., 1996). The structure of CRP2, a second heat-stable chromatin. remodeling protein of Drosophild which appears to be able to substitute in a mutant knocking out CRP 1, is currently under investigation. Very large aggregates have been observed.